Zoom lenses generally are formed of a plurality of groups (units) of one or more lens elements. It has been known to make three-unit zoom lenses with a positive front lens unit, a positive middle lens unit, and a negative rear lens unit. However, because of the desire to reduce the size, weight and costs of cameras with zoom lenses, efforts are being made to create more compact zoom lenses (also referred to as zoom lens systems) with a small number of lens elements and a minimum number of independent zooming lens units, while still producing high quality images over a large focal length range.
More specifically, a zoom lens system must maintain high image quality in each zoom position and retain a flat field over a range of field angles. In addition, it is desirable to have a minimum front vertex to film distance in the telephoto position. This provides a small package size and decreases the overall lens system volume. It is also desirable to have a minimum number of lens elements and a minimal number of separate zooming lens units in order to reduce cost and to decrease mechanical complexity. However, the requirement to produce a compact zoom lens system with a minimum number of independently movable zooming lens units and a minimum number of lens elements often conflicts with the requirement to produce high quality images over a large focal length range. This conflict occurs because when (i) the air spaces between lens units are reduced and (ii) the front vertex to the image plane distance is also reduced (provided that the optical power of the lens units does not change), the lens units can not move far enough during zooming. The lack of travel distance, in turn, decreases the lens system's zoom ratio. The zoom ratio ZR is measured by the ratio of lens system's focal lengths, i.e. ZR=f.sub.t /f.sub.w, where f.sub.w is the focal length of the zoom lens system in the wide-angle mode and f.sub.t is the focal length of the lens system in the telephoto position. If the optical power of lens units increases (without the additional lens elements) in order to provide a larger zoom ratio, lens surfaces become stronger, compromising the performance of the lens system.
Thus, as the optical powers of lens units increase to increase the zoom ratio to beyond 2.times., it becomes more difficult to maintain the lens system's performance. Because of this, lens systems with higher zoom ratios require larger numbers of lens elements. This, in turn, increases the cost of the lens systems and often makes the lens system less compact. As stated above, if the number of lens elements in a lens system is reduced, the performance of the lens system suffers.
The compactness of the lens system can be determined by the total length D of the lens system, measured from the front vertex of the lens system to its rear vertex. The smaller the value of D, the more compact the lens system.
The compactness of the zoom lens system can also be measured by the ratio L.sub.v to f.sub.t, where L.sub.v is the distance from the front vertex of the lens system to the film plane in the telephoto position. The smaller the ratio, the more compact the zoom lens system. However, as stated above, the compactness of the lens system should not come as a result of reduced zoom ratio or reduced optical performance.
The following five U.S. patents all describe zoom lens systems which have three lens units of positive, positive and negative optical power, respectfully.
U.S. Pat. No. 4,726,668, issued Feb. 23, 1988, discloses a zoom lens system with three lens units of positive, positive and negative optical powers, respectfully. An aperture stop is located within the middle lens unit. The front and the rear lens units move independently of each other while the second unit, which contains an aperture stop, is held stationary during zooming. This lens system has six lens elements and its total length D from the front vertex of the lens system to its rear vertex is 46.95 at the telephoto position. At the wide-angle position, the maximum amount of distortion is 5% and the amount of astigmatism is 0.8 millimeters. In the telephoto position, the distortion amount is again about 5% and the maximum astigmatism is greater than 0.8 millimeters. It is desirable to have a lens system that has better performance with respect to astigmatism and distortion, fewer lens elements and that is more compact, i.e. a lens system which has a substantially smaller total length D in the telephoto position.
U.S. Pat. No. 5,353,160, issued Oct. 4, 1994, discloses a zoom lens system which has either four or five lens elements arranged into three movable lens units. The lens system has a zoom ratio ZR=f.sub.t /f.sub.w of only 2.0, and its compactness ratio L.sub.v /f.sub.t is 0.93. The amount of distortion is about 2% to 4% at the wide-angle position and about 2% to 4% at the telephoto position for the various embodiments shown. The amount of astigmatism is about 0.4 millimeters in the wide-angle position and about 0.4 to 0.6 millimeters in telephoto position.
U.S. Pat. No. 5,325,235, issued Jun. 28, 1994, has relatively few lens elements (four). Its zoom ratio ZR is only 1.6 and its compactness ratio is about 1.0 (for example, it is 0.96 and 1.13 in the embodiments 3 and 5, respectfully). Thus, this zoom lens system is even less compact and has an even smaller zoom ratio than the zoom lens system described in U.S. Pat. No. 5,353,160. The maximum amount of astigmatism ranges from about 0.5 millimeters to about 1.0 millimeters in both the wide-angle and telephoto modes. Depending on the embodiment, the distortion varies from about 0.5% to about 2% when the lens system is in the telephoto position and from about 2% to about 5% when the lens system is in the wide-angle position.
U.S. Pat. No. 5,353,159, issued Oct. 4, 1994, discloses a zoom lens system with three lens units that move during zooming. The zoom lens system has a zoom ratio ZR of only 1.87, and a compactness ratio of about 1.01. This zoom lens system has 8 or 9 lens elements. The amount of distortion ranges from about 1% to about 4% in both the telephoto and wide-angle positions. The amount of astigmatism ranges from about 0.2 millimeters to about 0.5 millimeters when the lens system is in the wide-angle position and from about 0.2 to 0.7 millimeters when the lens system is in the telephoto position. The second zoom lens system embodiment disclosed in this reference has a lens element with a V-number of 70 in its second lens unit.
U.S. Pat. No. 5,216,547, issued Jun. 1, 1993, discloses a zoom lens system with three lens units that move during zooming. The zoom lens system has a zoom ratio ZR of only 1.87, and a compactness ratio of about 0.94. This zoom lens system has 7 lens elements. The amount of distortion ranges from about 2% to about 3% in both the telephoto and wide-angle positions. The amount of astigmatism or/and field curvature range from about 0.2 millimeters to about 0.5 millimeters when the lens system is in the wide-angle position and from about 0.2 to 0.7 millimeters when the lens system is in the telephoto position. The second zoom lens system embodiment disclosed in this reference has a lens element with a V-number of 70 in its second lens unit.
Other patents also provide a three unit lens system, but they are either substantially less compact than the lens systems of the above described U.S. patents, or have a substantially greater number of lens elements or have a substantially smaller zoom ratio.